I don't think it will be treated as 虐待

Since b doesn't make too much money, it's more than likely that a will have to take care of the kid completely by herself!
But if it were me, I will still get a divorce. There's no point living with such an assole, plus, if a make more money than b, then if they keep living together, it only resulted in a spending more money on b which is such a shame!